Petroleum, petrochemical, and chemical processing units generally use conventional control valves to control the large liquid and gas streams. The pressure loss and consequent energy loss across the control valve is substantial. The pressure drop across the control valve at the least open position for a stream with a flow rate of 2000 m3/hr could be about 172 kPa (25 psi). This represents almost 100 kW of dissipated power. As a result, the pump must be oversized to account for the energy dissipation, and that energy is lost on a consistent basis. Moreover, a flow sensing element needs to be installed in the system which adds to the installation cost. Finally, the control valve is typically sealed via a packing system to achieve the lower hydrocarbon fugitive emissions regulated by the EPA and other agencies.
Another problem associated with energy consumption in plants is the variable power requirements over time. For example, the energy required for the operation of various process units in the plant is often shared from one process stream to another, with the outside energy provided by a large energy source such as a fired heater. The process thermal energy may not be available for use for a period of time during start-up, and a supplemental energy source may be needed, such as a steam heater. The steam heater places a demand on the utility system that may not be required later during normal operation.
Typically, when power-recovery turbines are installed in process streams in petroleum, petrochemical, and chemical process plants, the power is sent through mechanical means to a driver for a pump or compressor because this generally has a higher efficiency than conversion to electricity through a generator. However, these systems have lower reliability than electrical generators, require that the power recovered matches closely with the power consumed in a single driver, and require a substantial amount of connection equipment in couplings, gears, switchgear, clutches, and the like that have to be located in a limited space. These issues have generally been addressed by generating electrical power at the power recovery point and providing that as fungible power for any driver within the process unit. Where the power recovery is greater than the power consumed within the process unit, it is exported into the higher voltage level on the plant grid. Most major power recovery installations seen to date have utilized this approach.
However, sending power to the plant's power grid presents its own set of difficulties. It requires electric power conditioning, voltage transformation, and frequency regulation. Equipment is now available in the power recovery field that can bring down unit cost to allow economic power recovery at lower power generation sources. When more cost efficient turbines are installed as control elements in process streams, multiple installations may be desirable. However, the power conditioning, transformer, electrical components, and safety equipment to protect the high voltage grid is costly for multiple installations.
In addition, the utility management of many refineries focuses on fuel gas, electricity, steam, and cooling water as distinct costs associated with operations. In techno-economic evaluations these sources are commonly normalized to a fuel gas equivalent cost to compare alternatives. At the design stage, many of these evaluations are done at a single design point. In reality, the plant will operate dynamically, and the operator will benefit from having choices that will allow optimization of each utility source more fully.
Therefore, there is a need for improved energy recovery and use in petroleum, petrochemical, and chemical processing units.